The present invention relates to a toner density detecting device for regularly controlling the toner density of a two-component developing agent used in a developing apparatus of an electronic copying machine or the like, and more specifically to a toner density detecting device for detecting a change in carrier density as a change of permeability and calculating the toner density on the basis of the carrier density.
FIG. 1 shows a prior art toner density detecting device 1. In FIG. 1, numeral 2 designates a developing device used in an electronic copying machine, 3 a magnet roller for carrying a developing agent in the developing device 2 and developing a charged pattern on a photoconductive drum 4, 5 a separating plate for clearing the magnet roller 3 of that portion of the developing agent returned to the developing device 2 without having been used in developing. The toner density detecting device 1 is provided with a detecting coil 6 as a detecting portion. In the developing device 2 constructed in this manner, a cover member 7 covering the detecting coil 6 is disposed so that the end face (hereinafter referred to also as head face) is in contact with the developing agent carried in a mobile manner on the magnet roller 3. The toner density is detected by measuring the inductance of the detecting coil 6 which depends on the permeability of the carrier of the developing agent.
An essential point here is the flowing condition of the developing agent touched by the head face. The permeability of the carrier of the developing agent in contact with the head face will change if the flow of the developing agent near the head face becomes uneven. In order to accurately measure the change of the carrier density of the developing agent as a change of permeability, therefore, it is necessary that the flow of the developing agent on the head face be uniform.
In the prior art toner density detecting device, however, the head face (end face of the cover member 6) is flat in shape, as shown in FIGS. 2A to 2C. Therefore, the developing agent is liable to flow unevenly, clogging the passage between the head face and the magnetic roller 3, as shown in FIG. 1. Thus, in measuring the inductance of the detecting coil 5, high-frequency noises may be produced to complicate accurate toner density measurement. FIG. 3 shows fluctuations in data on the measurement of the inductance.